JP2004-293660A published by the Japan Patent Office in 2004 (Japanese Patent Serial Number 3873192) proposes a front fork interposed between a front wheel and a vehicle body of a motorcycle for absorbing oscillations input into the front wheel from the road surface.
This front fork comprises a body-side tube that is connected to a handlebar of the motorcycle and fitted onto an outer circumference of a wheel-side tube connected to a front wheel axis of the same. The front fork incorporates a double-rod damper in the interior thereof.
The double-rod damper comprises a cylinder fixed in the wheel-side tube, a piston housed in the cylinder, an upper rod connected to an upper end of the piston, and a lower rod connected to a lower end of the piston. The interior of the cylinder is divided by the piston into an upper oil chamber and a lower oil chamber. These oil chambers are filled with working oil. An upper end of the upper oil chamber is closed by an upper partition, and a lower end of the lower oil chamber is closed by a lower partition.
The upper rod protrudes upward from the upper partition and is connected to an upper end of the body-side tube. The lower rod protrudes downward from the lower partition. An elongation/contraction space for the lower rod projecting from the lower partition is provided in the cylinder. The upper rod and the lower rod are formed to have an identical diameter. A suspension spring is interposed between the upper partition and the upper end of the body-side tube.
The upper oil chamber and the lower oil chamber communicate with each other via a damping valve installed in the piston. Further, a reservoir of working oil is provided in the interior of the body-side tube above the upper partition.
A space between the cylinder and the wheel-side tube is connected to the reservoir via a notch formed in the upper partition. Further, the elongation/contraction space of the lower rod under the lower partition is also connected to the space between the cylinder and the wheel-side tube via a notch formed in the lower end of the cylinder. These spaces are filled with working oil so as to function as a part of the reservoir.
A check valve is provided in the lower partition to allow working oil to flow from the reservoir into the lower oil chamber. Further, an air vent that connects the space between the cylinder and the wheel-side tube to the upper oil chamber is formed in an upper wall of the cylinder.
To fill the front fork with working oil when it is assembled, working oil is first poured into the reservoir and the front fork is then caused to elongate and contract. This operation makes working oil flow into the lower oil chamber from the reservoir via the check valve in the lower partition, and flow into the upper oil chamber via the damping valve. Air remaining in these oil chambers is then expelled to the outside of the cylinder from the air vent as working oil flows into the oil chambers.